1) Field
The embodiment relates to a quantum dot semiconductor device for use, for example, with an optical communication system, and particularly to a quantum dot semiconductor device suitable for use with a semiconductor optical amplifier (SOA) in which a quantum dot is used.
2) Description of the Related Art
In recent years, a semiconductor optical amplifier and a semiconductor laser get much attention in the field of optical communications because they are small in size and low in power consumption.
On the other hand, semiconductor devices such as semiconductor optical amplifiers or semiconductor lasers require an uncooled configuration to achieve further reduction of power consumption.
However, it is difficult to achieve an uncooled configuration in a quantum well semiconductor device wherein a quantum well is used in an active layer.
In particular, since a quantum well semiconductor device has a continuous gain band and has a temperature dependency of the carrier distribution, the gain varies greatly depending upon the operation temperature. On the other hand, if injection current is increased in order to inject sufficient carriers, then the power consumption increases. Further, since the device temperature rises by increase of the injection current, there is a limitation to the increase of the injection current as well. Therefore, it is difficult to achieve an uncooled configuration for a quantum well semiconductor device.
On the other hand, a conventional semiconductor device wherein a quantum dot is used in an active layer is not desirable to achieve an uncooled configuration because it is sensitive to the temperature. Therefore, a method has been proposed wherein p-type impurity layer is formed in an active region to shift the Fermi energy to the high energy side thereby to reduce the variation amount of the carrier distribution by variation of the device temperature so that the temperature characteristic of the gain is improved (refer to, for example, U.S. Pat. No. 6,859,477).
Also a method has been proposed wherein, in order to accelerate relaxation of carriers to a ground level of a quantum dot in a quantum dot semiconductor device, p-type impurity is included into an active layer (refer to, for example, Japanese Patent Laid-Open No. 2003-23219).
Incidentally, regarding a quantum dot semiconductor device, a method has been proposed which utilizes the fact that a plurality of quantum dots are non-uniform in size in order to expand the gain band so that broadband WDM (Wavelength Division Multiplexing) signals can be amplified collectively (refer to Japanese Patent Laid-Open No. 2003-124574).